Method For Producing High-Strength Aluminum Alloy Extruded Product

ABSTRACT

A method for producing a high-strength aluminum alloy extruded product includes casting a billet using an aluminum alloy containing, by mass %, 6.0 to 8.0% of Zn, 1.0 to 3.5% of Mg, 0.2 to 1.5% of Cu, 0.10 to 0.25% of Zr, 0.005 to 0.05% of Ti, and 0.5% or less of Mn, [Mn+Zr] being 0.10 to 0.60%, with the balance being Al and unavoidable impurities, homogenizing the billet and then extruding the homogenized billet without being cooled, cooling an extruded product immediately after the extrusion at an average rate of 70 to 500° C./min, and then performing artificial aging.

The present application incorporates the contents of Japanese patent application No. 2018-098403 filed on May 23, 2018 and Japanese patent application No. 2018-195032 filed on Oct. 16, 2018.

BACKGROUND

The present disclosure relates to a method for producing a high-strength aluminum alloy extruded product, and more particularly, to a method for producing a high-strength aluminum alloy extruded product that is effective in improving productivity thereof.

Al—Zn—Mg (7000 series) aluminum alloys are used for structural members of vehicles and structural parts of machines as such aluminum alloys tend to have high strength.

A technical problem of such 7000 series aluminum alloys is that recrystallization tends to occur on a surface of an extruded product during an extrusion, and consequently, the extruded product tends to have reduced stress corrosion cracking resistance. For that reason, it is necessary to manage various production conditions such as casting conditions of a billet used for the extrusion and cooling conditions after the extrusion.

JP-A-2014-145119 discloses a production method including a reversion treatment that includes heating a 7000 series aluminum alloy extruded product produced by press quenching to 200 to 550° C. at a rate of temperature rise of 0.4° C./second or more and subsequently cooling the heated extruded product at a cooling rate of 0.5° C./second or more, and further including performing a crushing. However, a production process of this method is long and consequently becomes a factor contributing to a decrease in productivity.

JP-B-2928445 discloses a production method including rolling or drawing after the extrusion. However, this method needs an additional processing step after the extrusion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a composition of each alloy used for an evaluation.

FIG. 2 illustrates production conditions of each extruded product evaluated.

FIG. 3 illustrates evaluation results.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, when a first element is described as being “connected” or “coupled” to a second element, such description includes embodiments in which the first and second elements are directly connected or coupled to each other, and also includes embodiments in which the first and second elements are indirectly connected of coupled to each other with one or more other intervening elements in between.

According to one of many different embodiments, a method for producing a high-strength aluminum alloy extruded product is provided, the method including casting a billet using an aluminum alloy containing, by mass %, 6.0 to 8.0% of Zn, 1.0 to 3.5% of Mg, 0.2 to 1.5% of Cu, 0.10 to 0.25% of Zr, 0.005 to 0.05% of Ti, and 0.5% or less of Mn, [Mn+Zr] being 0.10 to 0.60%, with the balance being Al and unavoidable impurities, homogenizing the billet and then extruding the homogenized billet without being cooled, cooling an extruded product immediately after the extrusion at an average rate of 70 to 500° C./min, and then performing artificial aging.

As a result, according to some embodiments, an aluminum alloy extruded product having high strength and excellent stress corrosion cracking resistance can be produced, and also productivity of the extruded product can be improved.

To produce the aluminum alloy extruded product, a cylindrical billet is cast, and this billet is loaded into a container of an extruder and extruded by pressure applied by a stem or the like.

Examples of the extruder includes a direct extruder and an indirect extruder, and these extruders are both configured to perform a hot extrusion.

When a cast billet is left after being produced by pouring a melt of the aluminum alloy into a mold, components contained in the alloy are solidified and appears as a microscopic product resulted from segregation in a cast structure.

To eliminate the segregation, it is necessary to perform homogenization (HOMO) on the billet to keep a billet temperature at 470 to 560° C. for 1 to 14 hours. In the conventional practice, the billet has been temporarily cooled after the homogenization and kept at a normal temperature.

Meanwhile, when subjected to the hot extrusion on the extruder side, the billet at room temperature has been preheated to 400 to 500° C. in a heating furnace and then loaded into the container of the extruder.

In the present disclosure, on the other hand, one of the features is that the cast billet is loaded straight into the container of the extruder for the extrusion without being cooled after the homogenization.

In the present disclosure, when casting the billet, a casting rate is preferably 50 mm/min or more and an average crystal grain size of a cast structure is preferably 300 μm or less.

When the average crystal grain size of the cast structure is small, there is the effect of preventing recrystallization of the surface of the extruded product during the extrusion.

The components of the aluminum alloy are selected described below, to prevent the recrystallization during the extrusion and to improve the stress corrosion cracking resistance as well as to enhance the strength.

To enhance the strength while keeping a decrease in an extrusion property small, Zn is set within a range of 6.0 to 8.0%, and 1.0 to 3.5% of Mg and 0.2 to 1.5% of Cu are added.

A Zn content is preferably 6.30 to 7.10%, more preferably 6.70 to 7.10%.

A Mg component contributes to strength enhancement. However, when the Mg component of more than 3.5% is added, the extrusion property is reduced. A Mg content is preferably 1.57 to 3.10%, more preferably 1.70 to 3.10%.

A Cu component contributes to strength enhancement by a solid solution effect. However, when the Cu component of more than 1.5% is added, corrosion resistance is reduced. A Cu content is preferably 0.2 to 0.5%, more preferably 0.25 to 0.5%.

It has been known that Mn, Cr, and Zr all have the action of preventing the recrystallization from occurring on a surface part of the extruded product during the extrusion. Among those three components, a Cr component has the highest quenching sensitivity, and thus press-end quenching by air-cooling is not sufficient and water-cooling is needed. Therefore, a Cr content is preferably 0.01%, more preferably 0%.

In the present disclosure, a Zr component that has not as high quenching sensitivity as Cr and has a strong recrystallization-preventing effect is added in an amount within a range of 0.10 to 0.25%. A Zr content is preferably 0.13 to 0.24%, more preferably 0.18 to 0.24%.

When the Zr content is more than 0.25%, it becomes difficult for Zr to fuse into a melt.

In the present disclosure, Mn that has quenching sensitivity between Cr and Zr may be added in an amount within a range of 0.5% or less. In this case, a total amount of [Mn+Zr] is set to be within a range of 0.10 to 0.60%. A Mn content is preferably 0.32% or less, and the total amount of [Mn+Zr] is preferably 0.13 to 0.56%, more preferably 0.20 to 0.56%.

Cr in the present disclosure is treated as an unavoidable impurity.

A Ti component is effective for micronization of crystal grains when casting the billet for the extrusion, and thus is preferably added in an amount within a range of 0.005 to 0.05%. A Ti content is preferably 0.03 to 0.04%.

In the present disclosure, other components except for the above-referenced components are unavoidable impurities, and Fe and Si components among those other components tend to get mixed into an aluminum alloy during its production process.

A content of the Fe component is preferably limited to 0.2% or less and a content of the Si component is preferably limited to 0.1% or less.

In the present disclosure, a billet is cast using an aluminum alloy having the above-referenced composition, the cast billet is loaded straight into a container of an extruder after the homogenization of the billet at 470 to 560° C., and the cast billet is extruded so that an extruded product has a temperature within a range of 500 to 585° C.

In this case, if a product temperature immediately after the extrusion is not high enough, satisfactory quenching (press-end quenching) cannot be performed by air-cooling immediately after the extrusion.

For that reason, the cast billet is extruded so that the product immediately after the extrusion has a temperature of 500° C. or more.

When the temperature is more than 585° C., a defect such as a gouge tends to be formed on external appearance of the extruded product. A temperature for performing the homogenization is preferably 480 to 520° C., and the product temperature immediately after the extrusion is preferably 505 to 530° C.

In the present disclosure, sufficient quenching can be performed by air-cooling immediately after the extrusion at an average cooling rate of 70 to 500° C./min.

As a means for air-cooling, air-cooling by a strong fan can be used, for example.

The average cooling rate refers to an average of cooling rates for cooling an extruded product down to 200° C. or less. The average cooling rate of 70° C./min or more allows a subsequent artificial aging time to be shortened, thereby leading to an improvement in productivity corresponding thereto. The average cooling rate is preferably 250 to 350° C./min, more preferably 250 to 300° C./min.

Accordingly, the artificial aging time can be made shorter than a conventional artificial aging time that has been generally needed to obtain high strength. The artificial aging is preferably a two-stage heat treatment in which first-stage heat treatment conditions may be 80 to 130° C. and 7 hours or less and second-stage heat treatment conditions may be 130 to 160° C. and 13 hours or less. The first-stage and second-stage heat treatment conditions are preferably 110 to 120° C. and at 140 to 150° C., respectively.

By doing so, a total heat treatment time for the artificial aging is within 20 hours.

When the extruded product is produced as described above, a high-strength extruded product with a surface having a recrystallization depth of 150 μm or less, a tensile strength of 400 MPa or more, and a 0.2% proof stress of 380 MPa or more is produced.

Such a high-strength extruded product also has excellent stress corrosion cracking resistance.

By appropriately adjusting an amount of Zn, Mg and Cu added and adjusting an amount of Zr and Mn added in the aluminum alloy, and controlling an average crystal grain size of a cast structure to 300 μm or less, high strength of proof stress of 380 MPa or more is obtained by the press-end quenching by air-cooling and excellent stress corrosion cracking resistance is further obtained by reducing the recrystallization depth on the surface.

Utilization of the billet homogenization step for the preheating step at the time of the extrusion allows a heat treatment time after the press-end quenching to be shorter than that of a conventional high-strength aluminum alloy, thereby improving productivity.

A billet is cast using a melt having an aluminum alloy composition listed in the table of FIG. 1, an extruded product is produced in production conditions listed in FIG. 2, and the comparative evaluation results are listed in FIG. 3.

A melt each adjusted to have a composition listed in Examples 1 to 8, Examples a to f, and Comparative Examples 9 to 21 in the table of FIG. 1 was prepared, and a cylindrical billet was cast at a casting rate listed in the table of FIG. 1.

A measurement result of an average crystal grain size of the cast billet was listed as CRYSTAL GRAIN SIZE OF BILLET STRUCTURE (μm) in the table of FIG. 1.

As a method for evaluating the billet structure, a sample surface was polished to mirror finish and then etched with Keller's reagent (0.5% HF), and the average crystal grain size was determined from an image taken by an optical microscope at a magnification of 100 times.

Next, the billet was homogenized and extruded in the condition listed in FIG. 2.

In Examples 1 to 8 and Examples a to f, the billet was subjected as it was to the extrusion without being cooled after the homogenization.

This is described as “NOT HEATED” in the column of “HEATED OR NOT HEATED” of “BILLET TEMPERATURE” in the table of FIG. 2.

As for the billet that had been cooled down to room temperature after the homogenization and that was preheated before being loaded into the extruder, a preheating temperature thereof was listed in the column of “HEATED OR NOT HEATED” of “BILLET TEMPERATURE”.

Immediately after the extrusion, the air-cooling by a fan (press-end quenching) was performed at an average cooling rate listed as COOLING RATE AFTER EXTRUSION in the table of FIG. 2.

The average cooling rate was set within a range of 70 to 500° C./min. However, the average cooling rate is preferably fast and is preferably 200° C./min or more to shorten the heat treatment time in the subsequent artificial aging.

The extruded product that has been cooled in the above conditions was subjected to two-stage artificial aging in the heat treatment conditions listed in the table of FIG. 2.

The evaluation results are listed in the table of FIG. 3.

As for mechanical properties, a JIS No. 5 tensile test piece according to JIS Z2241 was prepared, and a tensile test was conducted in accordance with the JIS standards.

The stress corrosion cracking resistance (SCC resistance property) was determined using a test piece to which a stress equivalent to 80% of the proof stress was applied. The test piece with no crack being formed after going through 720 cycles, with the following testing condition defined as 1 cycle, was evaluated as achieving a target stress corrosion cracking resistance. 1 cycle

The test piece was immersed in a 3.5% NaCl aqueous solution at 25° C. for 10 min, then left to stand for 50 min at a temperature of 25° C. and a humidity of 40% and then naturally dried.

The number of the cycles taken until the crack was formed is listed in the table of FIG. 3.

As for a recrystallization depth on a surface of the extruded product, a cross section of the extruded product was polished to mirror finish and etched with a 3% NaOH aqueous solution, and an average recrystallization depth was measured from an image taken by an optical microscope at a magnification of 100 times.

In Examples 1 to 8 and Examples a to f, a billet was cast using a melt having a prescribed composition at a prescribed casting rate, the cast billet was extruded with an extruder without being cooled after the homogenization, and immediately after that, the extruded product was subjected to press-end quenching and two-stage artificial aging. Therefore, an extruded product having high strength and excellent stress corrosion cracking resistance (SCC resistance property) was produced.

In Comparative Examples 9 to 18, on the other hand, the cast billet was temporarily cooled down to room temperature after the homogenization, and then the billet was preheated and extruded. As the extra process described above was included, these Comparative Examples were less productive.

In Comparative Examples 19 and 20, the billet was extruded without being cooled after the homogenization. However, the composition of each alloy was out of the range of the present disclosure, and thus these Comparative Examples failed to achieve the target stress corrosion cracking resistance.

More specifically, the Cu component contained in Comparative Examples 19 and 20 was more than 1.5%, and Cr was further added to Comparative Example 20.

In Comparative Examples 17 and 18, an amount added of Zr was less than 0.10%, and these Comparative Examples failed to achieve the target stress corrosion cracking resistance.

In Comparative Examples 9 to 16, the billet was temporarily cooled down to a normal temperature after the homogenization and then sufficiently preheated before being loaded into the extruder, and thus although the quality of the extruded product such as high strength and stress corrosion cracking resistance is ensured, performing this process results in a decrease in productivity and an increase in cost corresponding to the required preheating. 

What is claimed is:
 1. A method for producing a high-strength aluminum alloy extruded product, comprising: casting a billet using an aluminum alloy containing, by mass %, 6.0 to 8.0% of Zn, 1.0 to 3.5% of Mg, 0.2 to 1.5% of Cu, 0.10 to 0.25% of Zr, 0.005 to 0.05% of Ti, and 0.5% or less of Mn, [Mn+Zr] being 0.10 to 0.60%, with the balance being Al and unavoidable impurities; homogenizing the billet and then extruding the homogenized billet without being cooled; cooling an extruded product immediately after the extrusion at an average rate of 70 to 500° C./min; and then performing artificial aging.
 2. The method for producing a high-strength aluminum alloy extruded product according to claim 1, wherein in casting the billet, a casting rate is 50 mm/min or more and an average crystal grain size of a cast structure is 300 μm or less.
 3. The method for producing a high-strength aluminum alloy extruded product according to claim 2, wherein the artificial aging is a two-stage heat treatment with a total heat treatment time of 20 hours or less, the two-stage heat treatment including a first stage performed at 80 to 130° C. for 7 hours or less and a second stage performed at 130 to 160° C. for 13 hours or less.
 4. The method for producing a high-strength aluminum alloy extruded product according to claim 3, wherein the high-strength aluminum alloy has a tensile strength of 400 MPa or more and a 0.2% proof stress of 380 MPa or more. 